Make more Ifs unreachable (#7094)

Previously the only Ifs that were typed unreachable were those in which
both arms were unreachable and those in which the condition was
unreachable that would have otherwise been typed none. This caused
problems in IRBuilder because Ifs with unreachable conditions and
value-returning arms would have concrete types, effectively hiding the
unreachable condition from the logic for dropping concretely typed
expressions preceding an unreachable expression when finishing a scope.

Relax the conditions under which an If can be typed unreachable so that
all Ifs with unreachable conditions or two unreachable arms are typed
unreachable. Propagating unreachability more eagerly this way makes
various optimizations of Ifs more powerful. It also requires new
handling for unreachable Ifs with concretely typed arms in the Printer
to ensure that printed wat remains valid.

Also update Unsubtyping, Flatten, and CodeFolding to account for the
newly unreachable Ifs.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/passes/CodeFolding.cpp b/src/passes/CodeFolding.cpp
index 42331b747..305eb1278 100644
--- a/src/passes/CodeFolding.cpp
+++ b/src/passes/CodeFolding.cpp
@@ -234,6 +234,13 @@ struct CodeFolding
     if (!curr->ifFalse) {
       return;
     }
+    if (curr->condition->type == Type::unreachable) {
+      // If the arms are foldable and concrete, we would be replacing an
+      // unreachable If with a concrete block, which may or may not be valid,
+      // depending on the context. Leave this for DCE rather than trying to
+      // handle that.
+      return;
+    }
     // If both are blocks, look for a tail we can merge.
     auto* left = curr->ifTrue->dynCast<Block>();
     auto* right = curr->ifFalse->dynCast<Block>();